Amplifying arrangement comprising thermionic tubes having more than one grid



. K. POSTHUMUS AMPLIFYING ARRANGEMENT COMPRISING THERMIONIC June 7, 1938.

TUBES HAVING MORE THAN ONE GRID- Filed April 19, 1934 INVENTOR U m M m ATTORNEY Patented June 7, 1938 AMPLIFYING ARRANGEMENT COMPRISING THERMIONIG TUBES HAVING MORE THAN ONE GRID Klaas Posthumus, Eindhoven, Netherlands, assignor to Radio Corporation of America, a corporation of Delaware Application April 19,

1934, Serial No. 721,295

In Germany April '7, 1933 2 Claims.

This invention relates to circuitv arrangements for amplifying high frequency oscillations in which thermionic tubes are used having more than one grid, for instance, screening grid tubes.

As is well known a reaction of the anode circuit on the controlling grid circuit and of the other grid circuits on the controlling grid circuit can occur with thermionic tubes having more than one grid. This reaction is due to the capacities of the various electrodes of the thermionic tubes with respect to each other. It. manifests itself the more as the frequency of the high frequency oscillations to be amplified is higher and it may eventually acquire such a value that the arrangement begins to self-oscillate. If the anode-controlling grid capacity is made very small as is the case for instance, with screening grid tubes,

the reaction of the anode circuit on the controlling grid circuit caused by this capacity will be small so that it may be neglected. In spite thereof a certain reaction of the anode on the controlling grid circuit will still occur also with screening grid tubes, if the frequency of the alternating currents to be amplified is very high, viz. through the anode-screening grid capacity and the screening grid-controllinggrid capacity. This would be impossible, it is true, if the screening grid were short-circuited for high frequency oscillations with the cathode. However, the connecting wires required for connecting the screening grid to the cathode, which wires are partly provided within and partly outside the envelope of the tube, have a certain self-induction, so that a high frequency alternating tension will be set up between the screening grid and the cathode, which tension acts more and more through the screening grid-controlling grid capacity on the controlling grid circuit as the frequency is made higher and the impedance in the anode circuit becomes larger.

Besides being coupled through the screening grid-controlling grid capacity, the screening and the controllinggrid circuits are coupled together through the mutual self-induction of the screening grid and of the controlling grid, by which a reaction of the first mentioned circuit on the last mentioned circuit is also caused.

Both electrostatic and inductive couplings of other electrodes with the controlling grid are present not only in screening grid tubes but also in other tubes having a plurality of grids. The invention has for its purpose to provide means by which the reaction in tubes having a plurality of grids in consequence of the said causes is avoided.

According to the invention this is achieved by establishing a high frequency connection between the end of the impedance in the anode circuit re-- mote from the anode and a grid which is not used as a controlling grid, in such a manner that the part of this connection which is common to the circuit connecting the said grid to the cathode, has a small impedance for high frequency currents.

The invention will be more clearly understood by reference to the accompanying drawing, representing by way of example, two embodiments thereof. In the drawing:'

Figure 1 shows a circuit arrangement for amplifying high frequency oscillations in which a screening grid tube is used; and,

Figures 2 and 3 are schematic amplifier circuit arrangements showing different embodiments of my invention.

The screening grid tube comprises an anode A, a screening grid S, a controlling grid G and a cathode K. The latter may be heated either directly or indirectly. For the sake of clearness all sources of direct tension or potential have been left out in Fig. 1, since only high frequency currents are considered. The anode circuit of the screening grid tube comprises an impedance Za constituted, for instance, by a resonant circuit consisting of an inductance La and a condenser Ca. The controlling grid circuit comprises an im- 3 pedance Zg consisting, for instance, of an inductance Lg and a condenser Cg. Furthermore, the anode-screening grid capacity is denoted in the drawing by Gas and the screening grid-con trolling grid capacity by Csg. The wire connecting the screening grid to the cathode which may be provided partly within and partly outside the envelope of the tube, has a definite self-induce tion L5 depending on the wire length. This selfinduotion is connected in series with the anodescreening, grid capacity Gas to form a path in parallel with the impedance Za inserted in the anode circuit. v

In the case represented in Figure 1 in which Z5. consists of a resonant circuit, the current traversing the capacity Gas and the inductance Ls can become large and even greatly exceed in strength the anode current passing through the tube. This results from the fact that the capacity Cas forms a part of the oscillatory circuit and at high frequencies the current passing through Gas and Le will usually be many times greater than the total current passing through the tube anode. If the frequency of the alternating currents to be amplified is high is the case Twith screening grid" transmitting tubes, the voltage .dropcaused by the'current through the inductance I45 of the connecting wire,

will also become large. The consequence of this "fluctuating voltage' set up'between' the screening grid and the" cathode is a 'reac'tionof the screen:

grid capacity 65,; on thecontrolling'grid circult. With an increasingfrequency of the alternating currents to be amplified this reaction manifests itself moreand more, so that the amplifying arrangement has eventually atendency V to self-oscillate. r V.

According to the invention this risk is avoided with the arrangement shownin' Fig. .2, inwhich the end of the impedance Za'i-n' the anode circuit remote from the anode is not immediatelycom" nected forhigh frequency oscillationsi to the cathodeK but toa terminal P. 'I'his terminal which may. lie outside the envelope of the tube is so arranged that'onlyl a-very short connection is required between it and the screeninggrid;

r It will be appreciated. that in the arrangement representedin Figure 2 alternating currents traversing-the connectingrwire lll from the terminal'P, consequently frointhe screening grid,

to the catho'de' K are much smaller, since this connecting (wire .no longer forms part of the resonance circuit Z a, which was the case in' the, ,arrangement"represented in'Figure l The traversing-current will-correspond to the sum of the anode alternating current and the screening grid alternating current. If the resonantcircuit 1Z9, is tuned to the frequency of the alternating V currents tobe .amplifiedythen the anode current will be in phase with the alternating current or voltage impressedon the controllinggrid. 'This' also holds good'forthe screening'grid alternat 40; mg current Sothat' 'the alternating current component passing through lead I0:is many times smaller than the "circulating current of the resonance circuit and therefore the potential of point P will not fluctuate greatly with refer-g 45; ence to the cathodeand. as the lead ais" made as short as feasible, the potential of the screengrid will fiuctuate'fonly a small amount" and v grid. "The small voltage drop through wire 1' is v in part due to the verylow self-inductance of this lead.

' I-Iitherto' the coupling occurringbetween contrQIand 'screening'gridsdue to their mutual selfinductionwas disregarded. However, this coupling also will'ma'nife'st itself atla suif ciently high'frequency of the alternating currents tobe ampli fied by the tube and increase with amplifying arrangements the tendency to self oscillate.

By using the arrangement represented in Fig. 3 l this'reaction also can be rendered substantially ineffective in addition to the reaction caused by the capacity between screen grid'and control grid.

7 In this figure the screening grid S isconnected V not'only to a terminal]? through a short con-i ing grid through the screening grid-controlling necting wired, but also to a. terminal Q through I a connecting'wire b. The terminal Q is main- 1 I tained at the same radio frequency potential as the low potential end of cirouit Zw by the direct right of the point R by which field the inductive 1 effect of the screening grid on the controlling grid is substantiallyneutralized. V V

'By' arranging the. 'point'R. not exactly in the middle ofithe screening grid, but by spacing it apart therefrom, the average voltage of the screening grid can be kept constant with respect to the controlling-grid, owing to which also the electrostatic reaction through the capacity Cs decreases, V In the circuit shown in Fig. '3,as in that of Fig. 2, the large circulatory currents of IO connecting the cathodeandterminal P and thereby cause substantial voltage variations of r '20' the resonant circuit do not flow through-ithe leadj Qthe screen grid. On the contrary'thescreen grida potential remains substantially the same as that 25 f of the cathode'so that thereacti'on caused by the screen grid on the'control grid due to the 'capacity therebetweerrisminimized. g

Although in the description and in the'drawing theme of; the invention is; suggestedfor screening 7 grid tubes, it will be appreciated thatit mayalso be 'usedto advantage in oth'erjtubes having a: -plu rality of grids, for instance pentodes. s

Having described my invention, what I claim as 7 novel and desire to secure byLetters Patent is:

1.. In an amplifier arrangement, the coinbinae.

tion of a vacuum'tube having 'a'cathode-a'control grid and a plate, a secondgrid located between the control gridand plate, an input circuit con V n'ectedto said' control" grid'a'nd cathode,- ater minal located adjacent said'second grid, a short leadconne'cted between said terminal and said.

second grid, a connection from said terminal to the cathode; said connection having such 1m pedance which together with the anode-screen 'grid capacity'formapath of negligible impedance" I to high" frequencycurrents; andan output/circuit hencecause only slight reaction on the. control having its lowpotential' end directly connectedto said terminal. Y

I '2.An amplifying devm ampfism theeoom bination of a 'vacuum tube having'ra cathode, an

tially the same'frequen'cy as said input circuit having its high potential end connected to said anode, a conductor" directly connected between the screen grid-andcathode, 'and'a connection from the low potential end of tlie second resonant circuit to a point on said screen grid'conductor close to 'thescreen grid. Y

KLAAS POSTHUMUS; 

